The article characterizes the state and development of the security policy of NATO and the EU in the 21st century. The main provisions of the EU security system are disclosed. Attention is focused on the key aspects of the formation of a strategic partnership in the specified period. Challenges to European and global security that consolidate NATO and EU efforts to deepen cooperation are outlined. There are terrorism, Russian aggression, hybrid, and now full-scale war in Ukraine and the annexation of Crimea, the strengthening of the role of China, Brexit, climate change, 5G, cybercrime, the global remilitarization race, increasing defense budgets, the coronavirus pandemic, and others. It is noted that both the EU and NATO are looking for adequate tools for responding to these challenges and tools for adapting to the new state of affairs. It was emphasized that Russian aggression and an attack on a sovereign democratic state have become the biggest threat to European security. The modern architecture of European and world security largely depends on whether a collective action will be able to provide support for Ukraine, protect values and stabilize the existing world order. Documents, official publications, statements of leaders, etc. were analyzed. The main principles and directions of NATO-EU cooperation in the field of security, as well as the priorities of political-strategic and diplomatic cooperation, which ensures the coordination of plans, have been accumulated. The strategic partnership is based on strengthening cooperation on crisis management, enabling NATO to support the EU in crisis management and operations through the sharing of resources, operational planning and management. It was concluded that the closer convergence of EU and NATO security policies in 2016 was activated by the EU policy, which was aimed at strengthening Europe’s collective security through cooperation with its strategic partners, starting with NATO. It was noted that one of the key principles of cooperation between the EU and NATO is a single set of forces. This means that joint member states do not have separate armed forces for NATO and separate ones for the EU. That is why the EU and NATO are working to streamline the priorities and bring together the processes of defense planning, cooperation in civil and military operations, military exercises and the development of the defense industry. The senior leadership of the EU and NATO also coordinates political activities and diplomatic matters to ensure a coherent and common approach in the Euro-Atlantic community. A separate important item in the list of joint measures between the EU and NATO is the improvement of military mobility, which should significantly contribute to the increase of joint defense capabilities. It is revealed how the intention to strengthen the partnership is reflected in the NATO 2030 development strategy and in the new strategic concept of NATO – 2022. It is concluded that according to the Strategy, the EU acts as an important and unique partner of NATO and that these organizations play a complementary, harmonious and mutually reinforcing role in maintaining international peace and security. Neither NATO nor the European Union has all the tools needed to respond to Russia’s malign activities or to challenges involving other non-military threats. Faced with these threats, they need to work together and have many tools, political, diplomatic, economic, military, which facilitate countermeasures. And despite the existence of different views on the role of the EU and NATO in the implementation of security policy, their mandates in the sphere of security and defense coincide and complement each other more and more. However, it is obvious that there are more common themes for NATO and the EU in the defense sphere than dividing factors.